Fuel-injected internal combustion engines fueled by liquid fuels, such as gasoline, diesel, and by alcohols, in part or in whole, such as ethanol, methanol, and the like, are well known. Internal combustion engines typically produce power by controllably combusting a compressed fuel/air mixture in a combustion cylinder. For spark-ignited engines, both fuel and air first enter the cylinder where an ignition source, such as a spark plug, ignites the fuel/air charge, typically just before the piston in the cylinder reaches top-dead-center of its compression stroke. In a spark-ignited engine fueled by gasoline, ignition of the fuel/air charge readily occurs except at extremely low temperatures because of the relatively low flash point of gasoline. (The term “flash point” of a fuel is defined herein as the lowest temperature at which the fuel can form an ignitable mixture in air). However, in a spark-ignited engine fueled by alcohols such as ethanol, or mixtures of ethanol and gasoline having a much higher flash point, ignition of the fuel/air charge may not occur at all under cooler climate conditions. For example, ethanol has a flashpoint of about 12.8° C. Thus, starting a spark-ignited engine fueled by ethanol can be difficult or impossible under cold ambient temperature conditions experienced seasonally in many parts of the world. The problem is further exacerbated by the presence of water in such mixtures, as ethanol typically distills as a 95/5% ethanol/water azeotrope.
In many geographic areas, it is highly desirable to provide some means for enhancing the cold starting capabilities of such spark-ignited engines fueled by ethanol or other blends of alcohol. There are currently several approaches to aid cold starting of such engines in cold ambients. For example, some engines are equipped with an auxiliary gasoline injection system for injecting gasoline into the fuel/air charge in cold ambient conditions. The use of such auxiliary system adds cost to the vehicle and to the operation of the vehicle and may increase the maintenance required for the engine.
Another approach to aid starting of spark-ignited engines, in cold ambient conditions, fueled by ethanol or other blends of alcohol is to pre-heat the fuel before being ignited in the combustion chamber. One such method is to provide a heat source, such as a thick film heater element, on the outside surface of a fuel injector body proximate the injector tip to pre-heat the fuel. The key to implementing this method is having sufficient heater power and heater surface area to transfer heat to the fuel. When electric current is passed through the electrically resistive material, heat is exchanged from the injector body to the fuel within the injector.
The amount of heat exchanged to the fuel within the injector is directly dependent on the heated surface area contacted by the fuel. Accordingly, it is advantageous to maximize the surface area contacted by the fuel. However, if the surface area of the heater is increased by increasing its diameter, the outside surface of the injector body needs to be increased too, which leads to an increased overall mass of the body. Notwithstanding the weight and size penalty associated therewith, if the overall mass of the body is increased, then the initial time delay to heat the fuel will also increase because the mass of the body has to be heated before its surface will heat the fuel.
Also, since a larger diameter fuel injector body causes an increased internal fluid volume, and the fuel itself is a relatively poor heat conductor, the larger volume of fluid does not transfer the heat well from the fluid near the heater surface area to the rest of the fluid. Moreover, the hollow valve assembly of prior art injectors allows fuel to pass through it, preventing the fuel passing through the valve assembly from picking up heat from the walls of the heated injector body.
Further, in prior art injectors, the heater is typically applied to the outside surface of the injector body, which is typically made of stainless steel. The heater is further typically overmolded with a plastic material in order to offer environmental protection to the electrical circuit. Stainless steel is known to be a poor heat conductor and, even when using a relatively thin injector body, most of the energy delivered by the heater is transferred to the external plastic overmold. Since the heat diffusivity of ethanol is very low, on the order of about 27 times below the one of stainless steel, this condition is worsened with the use of ethanol fuels.
What is needed in the art is a method to overcome the low heat diffusivity of ethanol and to increase the thermal efficiency of a heated fuel injector.
It is a principal object of the present invention to increase the area of the heated surface in contact with fuel flowing through the fuel injector to overcome the low heat diffusivity of ethanol fuels.
It is a further object of the invention to improve the heat transfer from the heated injector body to the fuel.